1. Field of the Invention
The present invention relates to an apparatus for indicating to the driver of a vehicle the air pressure in the pneumatic tires of the vehicle.
2. Description of the Prior Art
It is generally known that many road accidents are caused by tires being over-inflated or under-inflated. Over-inflated tires are the result of their overheating, being due either to excessive friction developing in the wheel assembly or to high vehicle speeds or to maintenance being made without due care. On the other hand, under-inflated tires are due to accidental puncturing, which is more likely with worn or low quality tires.
Unusual air pressure conditions in the tires, i.e. over or under-inflated tires, are dangerous and could be avoided by an automatic tire air pressure indicator which can operate as a road safety device. Furthermore, tires with proper air pressure could offer saving in tire cost, convenience, fuel economy and confidence for the drivers of motor vehicles and specially for those of long vehicles with twin tires.
Various devices have been developed in the prior art to detect and timely indicate to the driver of the vehicle, faulty air pressure in anyone of its tires. Basically the following principles of operation are used in those prior art devices:
a) The current collector method, wherein an air pressure gauge is mounted onto the rotating tire and an indicator at a nonrotating part of the vehicle, a current collector ring being used to connect the pressure gauge and indicator, where the current collector ring is fixed at the axle and a collector brush is fixed at the nonrotating part of the wheel assembly. PA0 b) The wireless method, wherein the tire rim is provided with wireless frequency transmitter and transmitting antenna, whereas a receiving antenna and receiver are provided at the nonrotating part of the wheel assembly. PA0 c) Various resonance circuit tire pressure detecting systems have also been developed in the prior art. A conventional resonance circuit system has been that wherein a detecting switch, an antenna and a circuit with inductance and capacitance is mounted onto the rotating tire and a transmitter and receiver at the nonrotating part of the vehicle wherein an intermittent retransmission of the abovementioned circuit occurs in response to the variations in tire air pressure, this retransmission being of the same frequency as the resonance frequency of the circuit.
Various disadvantages are associated with the aforementioned methods of the prior art, which have therefore not found broad application. The current collector ring method requires a sliding contact, wherein degradation of performance and lack of reliability tends to occur, sooner or later, caused by imperfect contact due to wear of the members, dust and mud. Further, the wireless method has a complicated structure necessitating mounting onto the rotating tire of a power source, e.g. battery, for the transmitter, which also makes the device unreliable and needs employment of severe maintenance standards.
Finally, the third conventional resonance circuit method also employs a complicated mechanism and tends to exhibit deviations in resonance, which also make the system unreliable. Tsagas EP-A-O 345 25199 disclosed an indicator of the air pressure in the tires of a vehicle, which utilized a pair of metallic annular strips, one strip being mounted on the rotating tire portion, and the other strip being mounted at a nonrotating part of the wheel assembly, a capacitive coupling occuring between these two strips, as a response to activating a tire pressure gauge detecting deviations from the predetermined normal tire pressure.
A disadvantage in the above Tsagas EP-A-O 345 199 system was that the tires had to be made with a special recession, within which fitted the abovementioned metallic annular strip, which was mounted at the rotating tire. Therefore application of this tire pressure detecting system was limited to tires of a particular design. Another disadvantage in the above Tsagas EP-A-O 345 199 system was that activation of the electronic circuit used to warn the driver of deviations from normal in the pressure of his tires, necessitated passage of electric current through the ball-bearing of the wheel shaft, wherein unpredictable cut-off of the warning signal occured due to the ball bearing lubricant.
Another disadvantage in the above, was that both the above mentioned metallic annular strips were exposed to rain water, mud, dust, etc, thereby resulting to undesired changes the capacitive coupling in between them.